The present invention relates to a process for the preparation of 2,2-bis(4-hydroxyphenyl)propane (hereinafter also referred to as bisphenol A) of high purity.
Bisphenol A is used as a raw material for polycarbonate resins or epoxy resins. The demand for polycarbonate resins employed for optical parts in particular has recently increased. As a result, colorless and highly purified bisphenol A is required compared to conventional bisphenol A.
Bisphenol A is prepared by the reaction of acetone with excess phenol in the presence of an acid catalyst or a combination of an acid catalyst and a cocatalyst such as sulfur compounds. The reaction mixture contains, in addition to bisphenol A, catalyst, unreacted acetone, unreacted phenol, water and other by-products formed by the reaction.
Principal components of the by-products are 2-(2-hydroxyphenyl)-2-(4-hydroxyphenyl)propane, 2,2,4-trimethyl-4-(4-hydroxyphenyl)chroman, trisphenol, polyphenol and unfavorable colored substances. These by-products deteriorate the properties of the resin prepared from bisphenol A.
As to the process for preparing bisphenol A of high purity from a reaction mixture, utilization of an adduct of bisphenol A and phenol is a well known technique for those who are skilled in the art. For example, catalyst, water and a part of the phenol are removed from the reaction mixture and the remaining liquid is cooled to crystallize bisphenol A as the adduct with phenol. The crystals are separated from the mother liquor containing by-products of the reaction. Phenol is removed from the adduct to recover bisphenol A. Various methods such as distillation, extraction, steam stripping and the like have been proposed for removing phenol. For example, Societa Italiana Resine S.p.A. has disclosed a method for vaporizing the adduct for 0.1 to 30 minutes at a temperature higher than 180.degree. C. under reduced pressure and fractionally condensing bisphenol [Japanese Patent Publication TOKKOSHO 52-42790 (1977)]. Mitsui Chemicals Co. has proposed a method for heating the adduct to 50.degree. C. or above by using a solvent having a boiling point of higher than 50.degree. C. to dissolve the phenol portion alone in the solvent [Japanese Patent Publication TOKKOSHO 36-23335 (1961)].
The intact bisphenol A obtained by those methods can be used as a raw material of common grade epoxy resins. The purity of the bisphenol A obtained by these methods, however, is insufficient for the preparation of polycarbonate. The crystals of adduct themselves contain impurities and thus bisphenol A obtained by removing phenol from the adduct has poor purity. Accordingly, further purification is needed in order to remove the impurities. For example, recrystallization from hot water is disclosed by Mitsui Petrochemicals Co. [Japanese Laid-Open Patent TOKKAISHO 59-62543 (1984)]. Washing by solvents is taught by General Electric Co. [Japanese Laid-Open Patent TOKKAISHO 59-231033 (1984)].
On the other hand, Union Carbide Corp. has disclosed a method for washing the adduct, which uses a mixture of phenol resulting from the decomposition of the adduct and fresh phenol externally supplied [Japanese Patent Publication TOKKOSHO 37-981 (1962) and 41-4454 (1966)]. No quantitative description, however, is found regarding the purity of the adduct obtained.
According to the examination by the present inventors, the hue of bisphenol A is not always satisfactory, even though its purity is acceptable. This is assumed to be due to the trace of washing liquid adhering to the resultant crystals of bisphenol A. As a result of further investigation, it has been found that commercial phenol can not provide satisfactory hue in the product bisphenol A, even though washing is repeated.